The specific aims of this proposal are to determine, 1) how effective the riboflavin analog 7,8-dimethyl-10-(3-chlorobenzyl) isoalloxazine [CBI] is as a K+ sparing diuretic and antihypertensive drug in the spontaneously hypertensive rat (SHR) when it is combined with hydrochlorothiazide (HCTZ) in various proportion, 2) if CBI therapy causes the rats to develop a functional primary hypothyroidism because of a post-receptor blockade of thyroid hormone action at the target cell level, 3) if CBI will inhibit the increased adrenal biosynthesis and secretion of corticosterone and aldosterone by ACTH in vivo, and 4)the loci at which CBI inhibits adrenocorticosteroid biosynthesis induced by Cortrosyn (ACTH 1-24), dibutyryl cyclic AMP (cAMP), angiotensin II (AII) and K+ in vitro by employing suspensions of isolated adrenal fasciculata and glomerulosa cells from SHR. In the first studies, the range of CBI doses will be from 0.1 mg to 0.5 mg per 100g body wt with either 100 mg or 200 mg of HCTZ/liter of drinking water. We will determine the optimal drug combination which will reduce the systolic blood pressure of the SHR, blunt the compensatory rise in the renin-angiotensin-aldosterone system, and spare K+ by assessing the concentration of K+ in the urine, plasma and iliopsoas muscle by flame photometry. Systolic blood pressures will be monitored during the twelve-week therapy period by the tail-cuff plethysmographic method, and plasma aldosterone concentration, plasma renin activity and plasma angiotensin II concentration will be measured by established RIA methods. Results of the dose-response studies for each parameter will be analyzed statistically using an analysis of variance and Dunnett's test for multiple variables. In studying the effects of CBI on thyroid hormone function and adrenal function, respectively, plasma TSH, free T4, T3, corticosterone and aldosterone will be assayed by established RIA methods after TRH and ACTH stimulations, respectively. The locus of blockade of adrenocorticosteroid biosynthesis will be determined by employing isolated adrenal glomerulosa and fasciculata cells from SHR treated with CBI and assaying for pregnenolone, progesterone, deoxycorticosterone, corticosterone and aldosterone by established RIA methods after ACTH, cAMP, AII and K+ stimulation in vitro. Since none of the rats treated previously showed evidence of toxicity, we are confident that we have discovered a safe and novel class of antihypertensive drugs which appear to function as dual-mechanism (inhibit aldosterone biosynthesis and action on Na+ reabsorption) diuretic agents with indirect K+ sparing properties.